1. Field of the Invention
This invention relates to resonant optical cavities and more particularly to ring-like cavities used in integrated optical devices.
2. Description of the Prior Art
An optical resonator is an important element, which can be incorporated into many of the components used for optical communication systems such as lasers, filters, routers, switches, etc. Such resonator can be easily realized in integrated optical devices with linear waveguides to form a planar lightwave circuit (PLC). One of the most common roles of the optical resonator is to serve as a wavelength dependent coupler between two (or more) waveguides (input/output (I/O) waveguides). This is schematically illustrated in FIGS. 1A and 1B, wherein a circular resonator serves for coupling between two linear waveguides. The light couples from one (input) linear waveguide into the resonator waveguide and from the resonator to the other (output) linear waveguide. In specific wavelengths, known as the resonance wavelengths of the resonator structure, all the light is eventually transferred from the first linear waveguide to the second linear waveguide. The resonator is typically characterized by following parameters:                Free spectral range (FSR);        Loss per revolution;        Coupling to the waveguides;        Q factor, which can be derived from the three parameters defined above.If the material of which the resonator is made of is “active” (i.e. able to provide optical gain), the resonator could operate as a laser, emitting light in the resonance frequencies of the device.        
Ring resonators in planar technology are generally comprise of a closed loop waveguide which is made of a material with a higher refractive index than its surrounding (see for example B. E. Little et al., “Vertically coupled glass microring resonator channel dropping filters”, IEEE Photonics Technology Letters vol. 11 no. 2, February 1999, p. 215–217). Here, ns is the refractive index of a substrate, ng, Wg and hg are the refractive index, width and height respectively of the input/output waveguides, nr, Wr and hr are the refractive index, width and height, respectively, of the ring, and n0 is the refractive index of a cladding layer as depicted in FIG. 1C.
Such implementation has several disadvantages. Characteristics such as large FSR and low loss are important for a micro-ring resonator, regardless the specific function it fulfills. However, achieving these characteristics simultaneously is difficult since the demands on the resonator shape conflict. To achieve large FSR resonator, the best design would be a circular ring with high refractive index contrast to achieve tight mode profiles. However, these characteristics would result in high losses which stem from three different mechanisms:                Bend radiation losses;        The smaller the core radius the higher the losses;        Core material absorption and scattering (material loss);        Surface scattering from the roughness of the core walls. These losses increase with the core-clad index contrast.        
There is therefore a need in the art to design a resonator structure that provides for large FSR and low loss/rev. The performance of a resonator-based device of any kind significantly depends on the resonator loss. There are two primary mechanisms that induce losses in the resonator, namely material loss and radiation loss. Material loss is an inherent property of the material comprising the resonator, and is an exponential function of the length of the resonator. As for the radiation loss, it arises from the waveguide imperfections such as surface roughness, and from the bend related radiation loss. The surface roughness is similar in effect to the material dependant loss and depends on the waveguide shape and refractive index. The bend related loss also depends on the waveguide shape and refractive index, but also critically depends on the radius of curvature. In order to decrease the radiation losses, what is needed is a resonator with large and smooth curvatures. This, however, would result in a very small FSR, and also would increase overall length of the resonator and increase losses related to the material and surface roughness loss. Hence, the current solutions cannot provide reduction of the loss of a resonator below a certain value.